Earth Planets Space, Vol. 57 (No. 6), pp. 491-505, 2005
Takeshi Nishimura1, Satoru Tanaka1, Teruo Yamawaki1, Hidekazu Yamamoto 2, Takashi Sano2, Minemori Sato1, Hisashi Nakahara1, Naoki Uchida1, Shuichiro Hori1, and Haruo Sato1
1Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
(Received December 3, 2004; Revised April 25, 2005; Accepted May 2, 2005)
We have examined temporal changes in seismic velocity of the crust through repeated active seismic experi-ments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.
Key words: Temporal changes, seismic velocity, Iwate vocano, active seismic experiment.